In addition to an optical recording medium, such as a reproduction-only DVD (digital versatile disc), a recordable DVD (e.g., DVD+RW, DVD+R, DVD-R, DVD-RW and DVD-RAM) is put to practical use.
The DVD+R and DVD+RW are on the way of improving the conventional technique for CD-R and CD-RW (a recordable compact disc) and for securing a reproducing compatibility of the DVD+R and DVD+RW with a reproduction-only DVD, the DVD+R and DVD+RW are so designed that a recording density (e.g., track pitch and signal mark length) and a thickness of the substrate are also compatible with those of CDs in order to attain the reproducing compatibility with DVD-ROMs. For example, the DVD+R is produced, like the CD-R, in such a manner that a substrate for recording the information in which a recording layer is disposed on a substrate and a reflective layer is disposed on the recording layer, is laminate with a substrate having the same form as the above-noted substrate for recording the information. In this case, the recording layer comprises a dye material.
With respect to the CD-R, the recording layer comprises a dye material and one of the characteristics of the CD-R is to have a high reflectance (65%) satisfying the CD standard; however, for obtaining a higher reflectance with the above-noted composition of the DVD+R, it is necessary that the recording layer can satisfy a specified complex refractive index at a wavelength of a light used for recordation and reproduction and the light absorption characteristics of the dye can satisfy the above-noted necessity. Also with respect to the DVD, the above-noted necessity is necessary.
With respect to the reproduction-only DVD, for enlarging the recording capacity, the reproduction-only DVD having two recording layers is proposed. For example, FIG. 1 is a cross-sectional view schematically showing a layers composition of the DVD having two recording layers. The first substrate 101 and the second substrate 102 are laminated to each other through the intermediate layer 105 comprising an UV-curing resin. On the inner surface of the first substrate 101, the first recoding layer 103 is disposed and on the inner surface of the second substrate 102, the second recoding layer 104 is disposed. The first recoding layer 103 is disposed as a semitransparent film of a dielectric film or the like. The second recoding layer 104 is disposed as a reflective film of a metal film or the like. In the first recoding layer 103, a recording mark in the form of concavo-convex is formed and by an effect of reflecting or interfering a reproduction laser, a recording signal is read. In the DVD shown in FIG. 1, the signal is read in the two recording layers, so that the DVD can obtain a memory capacity of 8.5 GB at most.
The first substrate 101 and the second substrate 102 have respectively a thickness of about 0.6 mm and the intermediate layer 105 has a thickness of about 50 μm. The first recording layer 103 as a semitransparent film is so disposed that the first recording layer 103 has a reflectance of around 30% and a laser light irradiated for reproducing the second recording layer 104 is damped in the first recording layer 103 through the reflection of about 30% of all light amount of the laser. Thereafter, the damped laser is reflected at the second recording layer 104 (as a reflective film) and after the laser is damped once again at the first recording layer 103, the laser gets out of the optical recording medium. By focusing the laser as a reproduction light so that the laser is brought to focus on the first recording layer 103 or the second recording layer 104 and by detecting the reflected light, a signal of each of the recording layers 103 and 104 can be reproduced. With respect to the DVD, the laser used for recordation and reproduction has a wavelength of generally about 650 nm.
However, up to date, with respect to a recordable DVD (e.g., DVD+R, DVD-R and DVD+RW), there is present only a recording medium having only one recording layer which can be read from one surface of the recording medium and for obtaining a larger memory capacity with using such an optical recording medium, it is necessary that the information can be reproduced and read from the both surfaces of the recording medium. This is because, since an optical recording medium in which recordation and reproduction are performed in two recording layers by irradiating a light to one surface of the medium, has two recording layers, when a signal is recorded by irradiating the laser which is brought to focus on the second recording layer (more distant from the optical pick up than the first recording layer), the laser is damped in the first recording layer, so that a disadvantage is caused wherein the light absorbance and the light reflectance in the second recording layer, which are required for recording the second recording layer cannot be compatibilized.
For example, Patent Literature 1 proposes an optical information medium which is constructed to be able to record in both two recording layers comprising an organic dye by irradiating the light to one surface of the optical information medium and the reproduction thereof can be performed also in both two recording layers by irradiating the light to one surface of the optical information medium. However, in this proposal, proposed is only an optical recording medium produced by laminating such two substrates to each other, as a first substrate in which recordation is performed by irradiating the laser to the surface of the substrate and a second substrate in which recordation is performed by irradiating the laser to the surface of the second recording layer, and the above-noted disadvantage in which the light absorbance and light reflectance in the second recording layer which are required for recording the second recording layer can be difficultly compatibilized, can not be solved. In this patent document, there is not disclosed a result of studying the recording signal with respect to the above-noted recording medium. Further, with respect to the second recording layer of the above-noted recording medium, since the layers composition around the second recording layer of the above-noted recording medium differs from the layers composition around the recording layer of a conventional CD-R or DVD+R, a disadvantage is caused wherein the recording mark can be difficultly formed in the second recording layer of the above-noted recording medium. This is because, since a conventional recording and reproducing medium having one recording layer comprises a substrate (to which the recording laser for the information is irradiated), recording layer (dye layer), reflective layer and protective layer in this order and the above-noted recording and reproducing medium having two recording layers comprises a laminating layer (to which the transmitted laser through the first recording layer is irradiated), inorganic protective layer, recording layer, reflective layer and substrate in this order, the recording mark forming environment (an adjacent layer of the recording layer) of the above-noted recording layer differs from that of the recording layer of a conventional recording medium. Therefore, with respect to the above-noted optical information medium disclosed in the above-noted patent document, there is a disadvantage wherein recording and reproducing properties, such as modulation factor and jitter, which correspond to a DVD system can be difficultly obtained.
Further, Patent Literatures 2 and 3 propose, with respect to the second recording layer of the recording and reproducing medium having two recording layers, an optical recording medium which comprises a laminating layer (to which the transmitted laser through the first recording layer is irradiated), inorganic protective layer, second recording layer, reflective layer and substrate in this order. However in these patent documents, there is disclosed only with respect to the presence of the recording mark (modulation factor) and the proposal thereof is the same proposal as that of the above-noted optical recording medium produced by laminating two substrates (first substrate in which recordation is performed by irradiating the light to the surface of the substrate and second substrate in which recordation is performed by irradiating the light to the surface of the recording layer) to each other, which is described in Patent Literature 1; therefore, by the above-noted proposal, the above-noted problem with respect to recordation and reproduction cannot be solved at all.
On the other hand, as a dye material used for disposing the recording layer of a DVD, a cyanine dye, azo dye and squarylium dye are put into practice. However, these organic dye materials have poor light resistance, therefore, in the case of disposing the recording layer using particularly a cyanine dye or squarylium dye, by using these dyes in combination with a light-resisting agent, such as an aluminum compound, a bisthiol metal complex and a formazane compound, the practical light-resistance of the recording layer is secured. For example, the squarylium metal chelate compound dye disclosed in Patent Literatures 4 and 5 is excellent in disc recording properties. However, for securing satisfactorily the light-resistance of the recording layer, it is proposed that the recording layer is disposed using the squarylium metal chelate compound dye in combination with various light-resisting agents (see Patent Literatures 6 to 9). However in this case, when a light-resisting agent is incorporated in the composition of the recording layer, the optical properties and thermal decomposition properties of the film as the recording layer are changed, thus a disadvantage is caused wherein while the practical light-resistance of the recording layer can be secured, the recording and reproducing performance of the recording layer is lowered.
Therefore, an optical recording medium comprising a first recording layer and a second recording layer; in which recordation and reproduction are performed in two recording layers and also in the second recording layer which is located more distant from the substrate to which the laser is irradiated than the first recording layer, advantageous recording signal properties can be obtained; and which is excellent in light-resistance, is not yet provided and the prompt development thereof is desired nowadays.
Patent Literature 1 Japanese Patent Application Laid-Open (JP-A) No. 11-66622
Patent Literature 2 Japanese Patent Application Laid-Open (JP-A) No. 2000-311384
Patent Literature 3 Japanese Patent Application Laid-Open (JP-A) No. 2003-331473
Patent Literature 4 International Publication No. WO01/44233
Patent Literature 5 International Publication No. WO01/44375
Patent Literature 6 Japanese Patent Application Laid-Open (JP-A) No. 2002-370451
Patent Literature 7 Japanese Patent Application Laid-Open (JP-A) No. 2002-370452
Patent Literature 8 Japanese Patent Application Laid-Open (JP-A) No. 2002-370453
Patent Literature 9 Japanese Patent Application Laid-Open (JP-A) No. 2002-370454